2007ApJ...669..776K

Using the Gemini Near-Infrared Spectrograph (GNIRS), we have assembled a complete sample of 20 K-selected galaxies at 2.0<z<2.7 with high-quality near-infrared spectra. As described in a previous paper, 9 of these 20 galaxies have strongly suppressed star formation and no detected emission lines. The present paper concerns the 11 galaxies with detected Hα emission and studies the origin of the line emission using the GNIRS spectra and follow-up observations with SINFONI on the VLT. Based on their [N II]/Hα ratios, the spatial extent of the line emission, and several other diagnostics, we infer that 4 of the 11 emission-line galaxies host narrow-line active galactic nuclei (AGNs). The AGN host galaxies have stellar populations ranging from evolved to star-forming. Combining our sample with a UV-selected galaxy sample at the same redshift that spans a broader range in stellar mass, we find that black hole accretion is more effective at the high-mass end of the galaxy distribution (∼2.9x10¹¹ M_☉) at z∼2.3. Furthermore, by comparing our results with SDSS data, we show that the AGN activity in massive galaxies has decreased significantly between z∼2.3 and 0. AGNs with similar normalized accretion rates as those detected in our K-selected galaxies reside in less massive galaxies (∼4.0x10¹⁰ M_☉) at low redshift. This is direct evidence for downsizing of AGN host galaxies. Finally, we speculate that the typical stellar mass scale of the actively accreting AGN host galaxies, both at low and at high redshift, might be similar to the mass scale at which star-forming galaxies seem to transform into red, passive systems.